As a system for cremation, a “roaster system” has been known. In the roaster system, cremation is performed by placing a coffin on a grid-form support called a fire grate. Since a space is created between the coffin and the bottom of a furnace by the fire grate, convection currents of air and heat smoothly flow in the furnace. Therefore, this system has an advantageously high combustion efficiency. However, the cremated bones are burned to the bottom through the fire grate for supporting the coffin. The burned bones (cremains) are collected by a heat-resistant steel plate or a catch pan formed of cast iron arranged in the furnace bottom; however, it is difficult to keep the form of human body.
Then, the present inventors proposed in Patent Document 1 that the fire grate is allowed to collapse during a cremation process. In the proposal of Patent Document 1, a low-melting point copper alloy is used as an element of a fire grate. When the element is heated, it is softened and melted. By this mechanism, the fire grate is allowed to collapse.
The proposal of Patent Document 1 is effective in that the cremains keep the form of human body; however, the copper alloy is mixed in the cremains. When the cremains and a metal different in color are present together, a bereaved family would feel uncomfortable when they pick up bones. Then, they proposed in Patent Document 2 to compose a fire grate of alumina-based hard ceramic having the same in color as white cremains. However, in the proposal of Patent Document 2, the fire grate does not collapse. Thus, burned bones go down through the fire grate and break into pieces.
Furthermore, Patent Document 3 discloses as follows. Since the shell coat layer for a cast has extremely high heat-resistance, the components thereof remain unchanged; however, the coat layer contains SiO2 as a main component. Thus, it deteriorates at high temperature and becomes collapsible. In breaking and removing the shell coat layer, since collapsibility of the coat layer is enhanced, it is easy to take out a cast itself from the casting mold. However, the document does not disclose how to control the collapse, which varies in accordance with heat capacity changes depending upon the size of a cast.
Moreover, Patent Document 4 discloses that dolomite is added to a surface coat layer of a mold because the mold sticks to a cast, making it difficult to separate them. In addition, the document discloses that when dolomite is endothermically decomposed by the temperature of molten metal for a cast, the properties of the surface coat layer is modified, enhancing collapsibility. However, no information is provided as to collapse of a member maintaining the strength of the mold and provided outside the surface layer.
[Patent Document 1]    Japanese Patent Application Laid-Open No. 59-142323
[Patent Document 2]    Japanese Patent Application Laid-Open No. 7-119938
[Patent Document 3]    Japanese Patent Application Laid-Open No. 4-253546
[Patent Document 4]    Japanese Patent Application Laid-Open No. 10-113745